Fabrication of three-dimensional platinum microstructures with laser irradiation and electrochemical technique

Article abstract of DOI:10.1016/j.electacta.2006.08.040

Three-dimensional (3D) platinum microstructures were fabricated by successive procedures: aluminum anodizing, laser irradiation, nickel/platinum electroplating, and removal of the aluminum substrate, the oxide films, and the nickel metal layer. Aluminum plates and rods were anodized in an oxalic acid solution to form porous type oxide films. The anodized specimens were immersed in a nickel electroplating solution, and then irradiated with a pulsed Nd-yttrium aluminum garnet (YAG) laser beam to remove the anodic oxide film with a three-dimensional XYZθ stage. The specimens were cathodically polarized in the nickel and a platinum electroplating solution to form the metal micropattern at the laser-irradiated area. The electroplated specimens were immersed in NaOH solution to dissolve the aluminum substrate and the oxide films, and then immersed in HCl solution to dissolve the nickel deposits. A platinum grid-shaped microstructure, a microspring, and a cylindrical network microstructure with 50-100 μm line width were obtained successfully.

Full text of DOI:10.1016/j.electacta.2006.08.040

Electrochimica Acta 52 (2007) 2352–2358
Fabrication of three-dimensional platinum microstructures with
laser irradiation and electrochemical technique
T. Kikuchia,∗, H. Takahashia, T. Marukob
a Graduate School of Engineering, Hokkaido University, N13, W8, Kita-Ku, Sapporo, Japan
b Furuya Metal Co. Ltd., R&D Group, Shimodate Daiichi Kogyodanchi 1915, Morisoejima, Chikusei, Ibaraki, Japan
Received 19 June 2006; received in revised form 9 August 2006; accepted 19 August 2006
Available online 25 September 2006
Abstract
Three-dimensional (3D) platinum microstructures were fabricated by successive procedures: aluminum anodizing, laser irradiation,
nickel/platinum electroplating, and removal of the aluminum substrate, the oxide films, and the nickel metal layer. Aluminum plates and rods
were anodized in an oxalic acid solution to form porous type oxide films. The anodized specimens were immersed in a nickel electroplating solu-
tion, and then irradiated with a pulsed Nd–yttrium aluminum garnet (YAG) laser beam to remove the anodic oxide film with a three-dimensional
XYZθ stage. The specimens were cathodically polarized in the nickel and a platinum electroplating solution to form the metal micropattern at the
laser-irradiated area. The electroplated specimens were immersed in NaOH solution to dissolve the aluminum substrate and the oxide films, and
then immersed in HCl solution to dissolve the nickel deposits. A platinum grid-shaped microstructure, a microspring, and a cylindrical network
microstructure with 50–100 ␮m line width were obtained successfully.
© 2006 Elsevier Ltd. All rights reserved.
Keywords: Aluminum; Platinum; Laser irradiation; Electroplating; MEMS
1. Introduction
platinum is needed in application for MEMS, but photolithogra-
phy is seldom applied to fabrication of three-dimensional (3D)
microstructures.
Platinum group metals (PGM) have been widely used in
many industries due to its chemical stability, high temperature
resistance, and catalytic reactivity [1–5]. Especially the plat-
inum metal with high chemical stability has been investigated
to fabricate microstructure such as microelectromechanical sys-
tems (MEMS) and micro-bio-devices. de Haro et al. worked on
fabrication of implantable microelectrode arrays using selec-
tive platinum coating, and obtained the microelectrode with
good adhesive and high corrosion resistance [6]. Kashimura
et al. fabricated electrodes with sub-10-nm gap using electron
beam lithography and conventional electroplating technique,
and this structure will be used for physical measurements of
single molecules or nanoparticles [7]. Micropatterning of plat-
inum electrode on silicon substrate for MEMS was achieved
by Zaborowski et al. using wet etching process with pho-
tolithography [8]. Recently more complicated microstructure of
The authors have been developing a new method for 3D
microstructure fabrication of metals and organic compounds
using laser irradiation and electrochemical technique [9–15].
In this technique, an aluminum specimen covered with porous
type oxide films was irradiated with a pulsed Nd–yttrium alu-
minum garnet (YAG) laser in a solution to remove the oxide film.
Then, electroplating was carried out for the formation of metal
or organic compound micropattern at the laser-irradiated area,
and the microstructure could be obtained by lifting off process.
Three-dimensional nickel microstructures, such as microspring,
3D network micostructure, microring, and microbellows were
fabricated by the processes described above [9]. A microactu-
ator with nitrocellulose/nickel/polypyrrole three-layer structure
was also obtained by electrolytic polymerization of conducting
polymer [11].
In the present investigation, the authors fabricated 3D plat-
inum microstructures by aluminum anodizing, laser irradiation,
electroplating, and lifting off.
∗
Corresponding author. Tel.: +81 11 706 7112; fax: +81 11 706 7881.
E-mail address: kiku@elechem1-mc.eng.hokudai.ac.jp (T. Kikuchi).
0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.08.040

T. Kikuchi et al. / Electrochimica Acta 52 (2007) 2352–2358
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Table 1
Composition of platinum electroplating solution
Chemicals
Concentration (M)
Pt(NH3)2(NO2)2
CH3COONa
Na2CO3
0.051
0.85
0.94
CH3COOH
For pH adjustment
2. Experimental
2.1. Specimens and pretreatment
Highly pure aluminum plate (99.99 wt.%, 0.35 mm thick,
20 mm × 18 mm with a handle, Nippon Light Metal) and com-
mercial aluminum tube (99.5 wt.%, 1.6 mm i.d., 2.0 mm o.d.,
35 mm long, Nilaco) were used as the specimens. These speci-
mens were degreased ultrasonically in C2H5OH for 10 min, and
then electropolished in 13.6 M CH3COOH/2.56 M HClO4 solu-
tion at 28 V and 280 K. After electropolishing, the specimens
were anodized in 0.22 M (COOH)2 solution at 293 K for 30 min
with a constant current of 100 A/m2 to form 9 ␮m thick porous
type oxide films. The anodized specimens were immersed in
0.029 M alizarin red S dying solution at 323 K for 5 min, and
then boiled in doubly distilled water for ts = 15–180 min to seal
the pores.
2.2. Immersion test of anodized specimen in platinum
electroplating solution
The anodized specimens were immersed in a platinum elec-
troplating solution (pH 9.0) for 120 min whose chemical com-
position is shown in Table 1. Structural changes of specimens
after immersion were examined by confocal scanning laser
microscopy (CSLM: 1SA21, LASERTEC).
Fig. 1. (a–d) Fabrication of three-dimensional platinum microstructures by
anodizing, laser irradiation, nickel/platinum electroplating, and dissolution of
aluminum substrate/oxide films/nickel deposits.
2.3. Fabrication of platinum microstructures
After laser irradiation and nickel electroplating, the spec-
imens were immersed in platinum electroplating solution,
and then cathodically polarized for 180–240 min at con-
stant current density of 50–200 A/m2 and 353 K to form
the platinum micropatterns. Some specimens were subjected
to platinum electroplating without nickel plating to exam-
ine the role of nickel deposits in the subsequent platinum
plating.
Finally, platinum-electroplated specimens were immersed in
3 M NaOH solution at room temperature for about 2 h to dissolve
the aluminum substrate and the anodic oxide films, and then
were immersed in 12 M HCl solution at room temperature for
5 min to dissolve the nickel deposits. The fabrication process
of the three-dimensional platinum microstructures is shown in
Fig. 1.
The anodized specimens were immersed in doubly dis-
tilled water or 0.31 M NiSO4/0.40 M H3BO3 solution (nickel
electroplating solution) at 293 K, and then irradiated with a
Pulsed Nd–YAG laser. The specimens were set in a focal posi-
tion or a defocused position 5 mm from the focal plane of
the laser beam that had passed through a beam splitter, an
iris diaphragm, a convex lens with 60 mm focal length, and
a quartz window. The laser beam has 532 nm wavelength,
8 ns pulse width, 10 Hz frequency, and <0.5 mrad beam diver-
gence (full angle). Details of the laser irradiation setup have
been shown elsewhere [11]. During the laser irradiation, the
specimens were moved at 80 ␮m/s with a PC-controlled XYZ-
stage, and rotated at 2.5–10.0◦/s with a θ stage to remove
the oxide film continuously from the aluminum substrate. The
laser-irradiated specimens were cathodically polarized in the
nickel electroplating solution for 1 min at constant potential of
−1.2 V (versus saturated KCl–Ag/AgCl) and 293 K. A plat-
inum plate was used as the counter electrode for the electro-
plating.
Structural changes of the specimen by laser irradiation, elec-
troplating, and lifting off were examined by CSLM and field
emission scanning electron microscopy (FESEM: JSM-6300F,
JEOL). In the observation of the vertical cross-section of speci-
mens, the specimens were embedded in epoxy resin and polished
mechanically.

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T. Kikuchi et al. / Electrochimica Acta 52 (2007) 2352–2358
3. Results and discussion
can be observed here due to dissolution of the anodic oxide
films in platinum electroplating solution with high pH. How-
ever, at ts = 180 min (Fig. 2c), there is no dissolved area of the
oxide films. Pore-sealing in boiling water causes the hydration
of oxide to seal the pores in the oxide films by the volume expan-
sion [16]
3.1. Effect of pore-sealing on film dissolution in platinum
electroplating solution
Fig. 2 shows CSLM two-dimensional (2D) contrast images
of the surface of anodized aluminum specimens with differ-
ent pore-sealing times of (a) ts = 15 min, (b) ts = 60 min, and (c)
ts = 180 min after immersion in platinum electroplating solution
for 120 min. It is clear that there are white and gray parts in
ts = 15 min (Fig. 2a) and 60 min (Fig. 2b). These white parts
correspond to anodic oxide films and the gray parts to alu-
minum substrate. The aluminum substrate with rough surface
Al2O3 + nH2O = Al2O3nH2O
(1)
The imperfections in the anodic oxide films decrease by the
volume expansion of the oxide films. In addition, a highly crys-
talline hydroxide layer is formed on the surface of the oxide
films, and the hydroxide layer is highly dissolution-resistant in
Fig. 2. CSLM images of the surface of the specimen pore-sealed for (a) ts = 15 min, (b) ts = 60 min, and (c) ts = 180 min after immersion in platinum electroplating
solution for 120 min.

T. Kikuchi et al. / Electrochimica Acta 52 (2007) 2352–2358
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Fig. 3. Change in cathodic current density, ic, with cathodic potential, Ec, in
platinum electroplating solution using platinum plate.
acid and alkaline solutions [17]. Thickness and the crystallinity
of the hydrated oxide layer increase with pore-sealing time, ta,
resulting in a high stability of the anodic oxide films in platinum
electroplating solution. The anodic oxide films after sealing for
ts = 180 min remained undissolved in platinum electroplating
solution until 240 min.
3.2. Effect of electroplating conditions on the fabrication of
platinum micropattern
Fig. 3 shows the cathodic polarization curve, cathodic current
density, ia, versus cathodic potential, Ec, in platinum electro-
plating solution at 353 K on a platinum plate. The ic value
increases as Ec becomes higher between −100 and −200 mV,
and then steady value of about 1 A/m2 until Ec = −550 mV, fol-
lowed by a linear increase with Ec. The steady value of ic at
−200 to −550 mV corresponds to diffusion-limited reduction
of dissolved oxygen in electroplating solution, while increase
in ic above Ec = −550 mV corresponds to electrodeposition of
platinum metal and hydrogen evolution.
Fig. 4a shows a CSLM contrast image of the surface of the
specimen irradiated with the laser at P = 1.0 mW in doubly dis-
tilled water to form a grid pattern. The black lines correspond
to the laser-irradiated area, and the white background to the
anodic oxide films that was not laser irradiated. It is clear that the
anodic oxide films are removed continuously with about 50 ␮m
line width. The periodic changes of the film-removed line width
are due to the scanning of the laser beam with circle shape.
Three-dimensional height images of the CSLM and the vertical
cross-section of the specimen showed that the oxide film was
completely removed by laser irradiation, and the aluminum sub-
strate is exposed to water at laser-irradiated area. Fig. 4b shows a
CSLM image of the surface of the specimen with platinum elec-
troplating at ic = 150 A/m2 for electroplating time, tc = 180 min
after laser irradiation in distilled water. Electrodeposited plat-
inum can be observed on all the laser-irradiated areas except for
Fig. 4. CSLM images of the surface of the specimen after (a) laser irradiation
in distilled water and (b) platinum electroplating.
the center region of the image. There are concaves at the center
region, suggesting no platinum deposition. The deposited plat-
inum is on the oxide films around the laser-irradiated area as
well as on the film-removed area. This overflow of platinum is
due to the spread of platinum onto the anodic oxide film from
the laser-irradiated area. Platinum micropatterning with contin-
uous metal lines could not be obtained by the direct platinum
electroplating under conditions with different tcs and ics.
Fig. 5 shows CSLM images of the surface of the specimen
after platinum electroplating at current density (a) ic = 50 A/m2,
(b) ic = 100 A/m2, (c) ic = 150 A/m2, and (d) ic = 200 A/m2 and
353 K for 180 min. Here, the specimens were irradiated with

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T. Kikuchi et al. / Electrochimica Acta 52 (2007) 2352–2358
Fig. 5. CSLM image of the surface of the specimen after laser irradiation in nickel electroplating solution, nickel electroplating, and platinum electroplating. The
current densities of platinum electroplating, ia, are (a) ia = 50 A/m2, (b) ia = 100 A/m2, (c) ia = 150 A/m2, and (d) ia = 200 A/m2.
the laser in the nickel electroplating solution, and then cathod-
ically polarized to deposit nickel at the laser-irradiated area
before platinum electroplating. The continuous platinum lines
can be observed at the laser-irradiated areas at ic = 50–100 A/m2
(Fig. 5a–c), and the line width is 50–60 ␮m, showing wider lines
at higher ic. Platinum deposits are composed of dome-shaped
grains and the grain size is lager at higher ic.
containing solution and the subsequent nickel electroplating
suppress the formation of hydroxide film at the film-removed
area. This is due to the chemical deposition of nickel nanopar-
ticles during laser irradiation and nickel thin layer formation
during short electroplating (Fig. 6b). Therefore, the nickel thin
layer may also suppress the dissolution of anodic oxide film
and aluminum substrate during platinum electroplating, result-
ing in the deposition of platinum continuous lines whose width
is very similar to that of film-removed area (Fig. 5a–c). The rea-
son why whole surface of the aluminum specimen is covered
with platinum metal layer at high current density is discussed
bellow. The current efficiency for platinum deposition under
the electroplating conditions in this investigation is 35–40%
[16], and platinum deposition is associated with hydrogen evo-
lution, which occurs at the interface between oxide films and
the metal substrate by penetration of proton through imperfec-
tions of the oxide films. At higher on current densities, hydrogen
evolution is more vigorous, and may cause the removal of the
oxide films from the aluminum substrate leading to the plat-
inum deposition on the whole surface of aluminum substrate
(Fig. 5d).
At ic = 200 A/m2 (Fig. 5d), all the surface is covered with plat-
inum metal with small particles, i.e. platinum metal is deposited
at non laser-irradiated areas as well as laser-irradiated areas.
Observation of the vertical cross-section of the specimen showed
that anodic oxide films were completely removed after platinum
electroplating at ic = 200 A/m2. The effect of nickel plating on
the deposition behavior of platinum is discussed bellow. The
non-uniform platinum electrodeposition in the case of laser
irradiation in distilled water (Fig. 4b) is considered to be due
to the formation of thin hydroxide film at the laser-irradiated
area after laser irradiation (Fig. 6a). The oxide films may have
fine imperfections locally. Platinum may be deposited through
these imperfections, leading to the non-uniform deposition of
platinum. On the other hand, the laser irradiation in Ni2+ ion

T. Kikuchi et al. / Electrochimica Acta 52 (2007) 2352–2358
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3.3. Fabrication of 3D platinum microstructures
Fig. 7 shows an optical micrograph of platinum grid-shaped
microstructure that was obtained by anodizing, laser irradiation,
nickel/platinum electroplating, and aluminum substrate/oxide
Fig. 6. Schematic models of (a) non-uniform platinum deposition and (b) uni-
form platinum deposition.
Fig. 7. Optical micrograph of the platinum mesh-like microstructure fabricated
byanodizingofaluminumplate, laserirradiation, nickel/platinumelectroplating,
and aluminum substrate/oxide film/nickel metal dissolving.
Fig. 8. CSLM and FESEM images of the 3D platinum microstructures: (a)
microspring and (b and c) cylindrical network microstructure.

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T. Kikuchi et al. / Electrochimica Acta 52 (2007) 2352–2358
films/nickel dissolution. It is clear from Fig. 7 that free-standing
platinum micro-grid can be obtained by the successive pro-
cedures described above. The platinum metal has 50 ␮m line
width, 190 ␮m gap intervals, and 10 ␮m thick. The deforma-
tion of platinum microstructure by removal of the aluminum,
the oxide films, and nickel metal layer cannot be observed,
and the line intervals of the microstructure are extremely
regular.
Fig. 8a and b shows CSLM image of 3D platinum microstruc-
tures fabricated by the process shown in Fig. 1 with commercial
aluminum tube. A platinum microspring with 2 mm spring diam-
eter and 110 ␮m metal line width is shown in Fig. 8a. The
pitch of coil of the microspring appears to be very uniform,
while the diameter to be slightly fluctuated. Fig. 8b shows a
cylindrical platinum network microstructure with 2 mm diame-
ter and 110 ␮m metal line width. This microstructure consists of
microspring described above, connected by four pillars, and the
line intervals of the microstructure are regular because the pillars
prevent deformation during immersion of NaOH and HCl solu-
tions. Fig. 8c shows high magnification FESEM image of the
cylindrical platinum network microstructure. The microstruc-
ture is composed of small platinum grain, and the thickness of
platinum layer is about 15 ␮m.
(2) Non-uniform platinum electrodeposition occurs on the
laser-irradiated area in platinum electroplating after laser
irradiation in distilled water, while uniform platinum
micropatterning can be achieved by electroplating after for-
mation of thin nickel metal layer on the aluminum substrate.
(3) A combination of the procedures of aluminum anodizing,
laser irradiation, nickel/platinum electroplating, and alu-
minum substrate, oxide films, and nickel metal layer dis-
solution enables fabrication of three-dimensional platinum
microstructures such as grid, microspring, and cylindrical
network.
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